1. Field of the Invention
This invention relates to the field of controls for automatic transmissions. More particularly, the invention pertains to acquisition, electronic storage, and use of information regarding current operation of a transmission for which a change in its operating state is scheduled to occur. The invention relates to correcting a reference speed at which transmission operation is scheduled to change, where vehicle speed is influenced by unknown axle ratios and tire sizes in the driveline.
2. Description of the Prior Art
Gear ratio changes in automatic transmissions occur electronically according to a schedule of gear shift speeds stored in electronic memory on the basis of current throttle position and vehicle speed. Control algorithms executed by a microprocessor initiate changes in the transmission operating state when a reference speed is reached. In certain operating conditions, the gear shifts occur on the basis of engine speed when the engine throttle is near a wide open throttle condition. Also, other operating conditions of the transmission than upshifts and downshifts, such as the engagement and disengagement of a torque converter clutch, occur on the basis of a reference shaft speed within the transmission.
The reference speed at which these and other operating conditions change is transmission output shaft speed. However, a sensor drivably connected to the output shaft produces a signal representing vehicle speed, but not output shaft speed. The possibility of deriving output shaft speed from vehicle speed and the additional cost of sensing output shaft speed operate against having both vehicle speed sensor and output shaft speed sensor located on the same shaft.
Motor vehicles are equipped with axle differentials that convert rotation of transmission output shaft parallel to the longitudinal axis of the vehicle to rotation of the axle shafts directed transversely. Generally, the differential mechanism reduces speed of its input to that of its output. This speed reduction or axle ratio varies for a particular motor vehicle, transmission, and driveline depending on preference of the vehicle owner and intended use of the vehicle. After first sale by the vehicle manufacturer, the rear differential axle ratio can be changed by replacing the original equipment. Also, the wheel sizes and tire sizes can vary during the service life of the vehicle at the discretion of the vehicle owner from those sizes used as original equipment.
These changes in axle ratio, wheel size and tire size affect the relationship between vehicle speed and output shaft speed. Nonetheless, the vehicle manufacturer determines the shift points and other changes in transmission operation on the basis of output shaft speed.
In the prior art, uncompensated changes that effect a relationship between vehicle speed and output shaft speed cause the shift points of the transmission to change from the shift points set by the vehicle manufacturer. For example, if the axle ratio is higher than the axle ratio at which the shift points are determined, gear ratio changes occur later and the vehicle operator senses lack of acceleration after the gear shift because the shift occurs when engine speed is higher than the engine speed where maximum engine torque results. If the axle ratio is lower than the axle ratio assumed to establish the shift points, gear ratio changes occur early and the vehicle operator senses a lack of acceleration because the shift occurs before maximum engine torque is reached.
In conventional hydraulically controlled transmissions, a hydraulic signal produced by a governor at output shaft speed is used as a basis for scheduling gear ratio changes. Therefore, in transmissions of this type, there is direct correlation between output shaft speed and vehicle speed.